To conserve the fresh water tank supply when boondocking, it would be a good arrangement to cross-connect the grey tank to the toilet flush supply via a 12V pump and appropriate valving and backflow protections. Anyone done this?

We use the movie line from "Meet the Fockers" :
"If it's yellow let it mello. If it's brown flush it down"
We use minimal water thru the toilet.
Shower is what gets us with kiddos, girls.

We added 58 gallons of fresh water capacity so we now have 129 gallons of fresh water for when we are dry camping.

I've wondered about cross connecting gray tanks but haven't needed to try it yet. On the 37RB we have a total combined capacity of 90 gallons gray.
Also wondered how messy it would be to use the macerator to pump gray water out a hose and into a toilet if ever needed. We have 64 gallons combined capacity in black tanks.

EDIT: I read OPs post again and my post pretty much has no use lol....sorry...
You can somewhat cross connect them very simply. It’s the same method boondockers and full timers use to flush the black tanks using the grey water instead of fresh water wasting.
This only works if you first dump the black tank (when using this method to flush) or the black tank is barely full and your grey tank is close to full (used to extend stay/dumping)

#1- install a Valterra twist on valve to your sewage dump port
#2- make sure the Valterra valve is closed and open your grey tank valve, this will allow the grey water to fill the dump pipes but keeps it from dumping out because your new external valve is still closed
#3- now open your black tank valve while still leaving the grey open too and the grey water in your lines and tank will rush into the black tank to equalize the pressure
#4- the black and grey valves are then closed after you’ve let the fluids equalize. You won’t get black water crossover because the grey water was already in the way and rushed into the black tank. The soapy grey water will help flush out the black tank when you do dump it later.

I do this in short intervals after I dump my black and it’s empty. Since we all know to dump black first anyway there is still a full grey tank. I’ll use same procedure and let grey water rush into the empty black for a minute or so then close grey and dump black. After a few of these grey water flushes you should be almost empty on grey and helped flush out your black (win, win).
You can only do this using the external added valve, otherwise you’d open your dump pipe cap and get a nasty surprise!

There are a few options; 1st, do your dishes in a dish pan, and dump it down the toilet, just make sure all the spoons have been removed first.

Another option, there is a gate valve you can purchase to put on at the sewer line connection. This way you can open the grey water tank, then open the black tank valve and let the two tanks equalize. Make sure to re-close the valves after equalizing.

We have stated at NPs that have no sewer service, and they told use we could dump our sewer down the latrine. You just need a small portable waste tank you can lift when full. I have I think a 7 or 10 gallon portable tank for this purpose.

I am camping 99% without hookups. I am using this gate valve mentioned above - amazing. I just bought 42 gallons tote + flojet waste pump. Over this coming weekend I will stay at campground where there is no dump station. I will test how it works to dump tote to toilet.

Iím most interested in eliminating the use of fresh water to operate the toilet. That is, adjust the front (input) side of the water balance. So maybe thatís not possible without some really elaborate plumbing mods. Otherwise just need to collect some of the grey water at the tank discharge and manually fill the toilet bowl with it. The procedure for diverting the grey to black on the back end is a great one!